The invention relates to the use of polyaniline or derivatives thereof for absorbing electromagnetic radiation, including microwaves, radar waves, infrared waves, visible light waves, and ultraviolet waves. The invention further relates to the use of the radiation absorbing polyaniline compositions to modulate another electromagnetic beam. The invention also relates to the modification of the electromagnetic response of polyaniline compositions by chemical or electrochemical means. The invention further relates to electronic and microelectronic devices based on the chemical and physical properties of polyaniline and its derivatives, and the control of the properties.
When a spectrum of radiant energy is directed into a sample of some substances, several things may happen to the energy: (1) it may pass through the sample with little absorption taking place and therefore, little energy loss. (2) The direction of propagation of the beam may be altered by reflection, refraction, or diffraction. Scattering of the beam by particulate suspended matter may also be involved. (3) The radiant energy may be absorbed entirely or in part. The absorption involves a transfer of energy to the medium, and the absorption process is a specific phenomenon related to characteristic molecular and electronic structures; the wavelengths of certain components of the radiation may be absorbed while others pass through essentially undisturbed, depending on the characteristics of the substance. Components of the radiation are absorbed if its energy matches that energy which is required to raise molecular or ionic components of the sample from one energy level to another. Those energy transitions may involve vibrational, rotational, or electronic states. After it has been absorbed, that energy may be emitted as fluorescence, utilized to initiate chemical reactions, or actually dissipated as heat energy.
When molecules interact with radiant energy in the visible and ultraviolet region, the absorption consists in displacing an outer electron in the molecule, although sometimes the energy of the far ultraviolet is sufficient to exceed the energy of dissociation of certain bonds.
The absorption of radiant energy is a highly specific property of the molecular structure, and the frequency range within which energy can be absorbed is specifically dependent upon the molecular structure of the absorbing material. The smaller the energy difference between the ground state and the excited electronic state, the lower will be the frequency of absorption (i.e., the longer the wavelength). Chemical compounds with only single bonds involving sigma-valency electrons exhibit absorption spectra only below approximately 150 millimicrons. In covalently saturated compounds containing heteroatoms, such as nitrogen, oxygen, sulfur, and halogen, unshared p-electrons are present in addition to sigma electrons. Excitation promotes a p-orbital electron into an antibonding sigma orbital, such as occurs in ethers, amines, sulfides, and alkyl halides. In unsaturated compounds absorption results in the displacement of pi-electrons. Molecules containing single absorbing groups, called chromophores, undergo electronic absorption transitions at characteristic wavelengths, and the intensity of the absorption will be proportional to the number of that type of chromophore present in the molecule. Marked bathochromic shifts (absorption at longer wavelengths) occur when --OH, --NH.sub.2, and --SH, for example, replace hydrogen in unsaturated groups.
It is desirable for certain applications to have a material whose radiation absorption characteristics and index of refraction can be easily and reversibly modulated. Various polymeric materials have been investigated including polyacetylene, polymethylacrylonitrile, pyrazoline, tetracyanoethylene, tetracyanonaphthoquinodimethane, tetracyanoquinodimethane, polydiacetylene, polypyrrole, poly(N-methyl-pyrrole), polyphenylene vinylene, and polythiophene. Some of these polymeric materials are known to exhibit photoresponsive effects, but the materials have deficiencies when considered for certain electromagnetic applications. For example, polyacetylene and polydiacetylene are nonaromatic, possess unacceptable absorption band gaps, have limited photoresponse, are air sensitive, generally cannot be derivatized, and are not readily soluble and therefore cannot be easily deposited as a thin film from solution. In addition, most materials previously investigated for electromagnetic radiation absorption are not readily tunable, i.e., the photoresponses of the materials cannot be reversibly modulated by an external source of energy.
Organic polymers have long been studied for electronic transport and, more recently, for optical properties. The first organic polymers prepared were electrically insulating with conductivities as low as 10.sup.-14 (ohms cm).sup.-1. The insulating properties are the result of all the electrons in the polymer being localized in the hybrid-atom molecular orbital bonds, i.e. the saturated carbon framework of the polymer. These insulators, which include polymers such as poly(n-vinylcarbazole), or polyethylene, have extremely large band gaps with energy as high as 10 eV required to excite electrons from the valence to the conduction band. Electrical applications of insulating organic polymers are limited to insulating or supporting materials where low weight and excellent processing and mechanical properties are desirable.
High electrical conductivity has been observed in several conjugated polymer or polyene systems. The first and simplest organic polymer to show high conductivity was "doped" polyacetylene In the "doped" form its conductivity is in excess of 200 (ohm cm).sup.-1. Although polyacetylene was first prepared in the late 1950's, it was not until 1977 that this polyene was modified by combining the carbon chain with iodine and other molecular acceptors to produce a material with metallic conductivity.
Polyaniline is a family of polymers that has been under intensive study recently because the electronic and optical properties of the polymers can be modified through variations of either the number of protons, the number of electrons, or both. The polyaniline polymer can occur in several general forms including the so-called reduced form (leucoemeraldine base), possessing the general formula ##STR2## the partially oxidized so-called emeraldine base form, of the general formula ##STR3## and the fully oxidized so-called pernigraniline form, of the general formula ##STR4##
In practice, polyaniline generally exists as a mixture of the several forms with a general formula (I) of ##STR5##
When 0&lt;y&lt;1 the polyaniline polymers are referred to as poly(paraphenyleneamineimines) in which the oxidation state of the polymer continuously increases with decreasing value of y. The fully reduced poly(paraphenyleneamine) is referred to as leucoemeraldine, having the repeating units indicated above corresponding to a value of y=1. The fully oxidized poly(paraphenyleneimine) is referred to as pernigraniline, of repeat unit shown above corresponds to a value of y=0. The partly oxidized poly(paraphenyleneimine) with y in the range of greater than or equal to 0.35 and less than or equal to 0.65 is termed emeraldine, though the name emeraldine is often focused on y equal to or approximately 0.5 composition. Thus, the terms "leucoemeraldine", "emeraldine" and "pernigraniline" refer to different oxidation states of polyaniline. Each oxidation state can exist in the form of its base or in its protonated form (salt) by treatment of the base with an acid.
The use of the terms "protonated" and "partially protonated" herein includes, but is not limited to, the addition of hydrogen ions to the polymer by, for example, a protonic acid, such as mineral and/or organic acids. The use of the terms "protonated" and "partially protonated" herein also includes pseudoprotonation, wherein there is introduced into the polymer a cation such as, but not limited to, a metal ion, M.sup.+. For example, "50%" protonation of emeraldine leads formally to a composition of the formula ##STR6## which may be rewritten as ##STR7##
Formally, the degree of protonation may vary from a ratio of [H.sup.+ ]/[--N.dbd.]=0 to a ratio of [H.sup.+ ]/[--N.dbd.]=1. Protonation or partial protonation at the amine (--NH--) sites may also occur.
The electrical and optical properties of the polyaniline polymers vary with the different oxidation states and the different forms. For example, the leucoemeraldine base, emeraldine base and pernigraniline base forms of the polymer are electrically insulating while the emeraldine salt (protonated) form of the polymer is conductive. Protonation of emeraldine base by aqueous HCl (1M HCl) to produce the corresponding salt brings about an increase in electrical conductivity of approximately 10.sup.10 ; deprotonation occurs reversibly in aqueous base or upon exposure to vapor of, for example, ammonia. The emeraldine salt form can also be achieved by electrochemical oxidation if the leucoemeraldine base polymer or electrochemical reduction of the pernigraniline base polymer in the presence of an electrolyte of the appropriate pH. The rate of the electrochemical reversibility is very rapid; solid polyaniline can be switched between conducting, protonated and nonconducting states at a rate of approximately 10.sup.5 Hz for electrolytes in solution and even faster with solid electrolytes (E. Paul, et al., J. Phys. Chem. 1985, 89, 1441-1447) The rate of electrochemical reversibility is also controlled by the thickness of the film, thin films exhibiting a faster rate than thick films. Polyaniline can then be switched from insulating to conducting form as a function of protonation level (controlled by ion insertion) and oxidation state (controlled by electrochemical potential). Thus, in contrast to, for example, the polypyrrole mentioned above, polyaniline can be turned "on" by either a negative or a positive shift of the electrochemical potential, because polyaniline films are essentially insulating at sufficiently negative (approximately 0.00 V vs. SCE) or positive (+0.7 V vs. SCE) electrochemical potentials. Polyaniline can also then be turned "off" by an opposite shift of the electrochemical potential.
The conductivity of polyaniline is known to span 10 orders of magnitude and to be sensitive to pH and other chemical parameters. It is well known that the resistance of films of both the emeraldine base and 50% protonated emeraldine hydrochloride polymer decrease by a factor of approximately 3 to 4 when exposed to water vapor. The resistance increases only very slowly on removing the water vapor under dynamic vacuum. The polyaniline polymer exhibits conductivities of approximately 1 to 5 Siemens per centimeter (S/cm) when approximately half of its nitrogen atoms are protonated. Electrically conductive polyaniline salts, such as fully protonated emeraldine salt [(--C.sub.6 H.sub.4 --NH-- C.sub.6 H.sub.4 --NH.sup.+)--Cl.sup.- ].sub.x, have high conductivity (10.sup.-4 to 10.sup.+2 S/cm) and high dielectric constants (20 to 200) and have a dielectric loss tangent of from below 10.sup.-3 to approximately 10.sup.1. Dielectric loss values are obtained in the prior art by, for example, carbon filled polymers, but these losses are not as large as those observed for polyaniline.
Polyaniline has been used to coat semiconductor photoelectrodes, to serve as an electrochromatic display material, and to suppress corrosion of iron.
While the preparation of polyaniline polymers and the protonated derivatives thereof is known in the art, it is novel herein to use these compositions for the attenuation of electromagnetic radiation, particularly microwaves, radar waves, infrared waves, visible waves, and ultraviolet waves. A need exists for a polymeric material which can be designed to absorb microwaves, radar waves, infrared waves, visible waves, and ultraviolet waves. In addition, a need exists for a method of absorbing the electromagnetic radiation to modulate another electromagnetic beam. A need also exists for a method for the modification of the electromagnetic properties of polyaniline compositions by chemical or electrochemical means.